| Summary: | Microbial biofilms located on the lumen surface of dental unit water lines have resulted in the
persistent and widespread microbial contamination of the outflowing water. The aims of this
study were to 1) evaluate the water quality delivered by dental units at the University of British
Columbia 2) evaluate the effect of plating and incubation conditions on heterotrophic plate
counts and 3) evaluate the effectiveness of marketed products in reducing microbial counts in
water used for nonsurgical and surgical dental procedures. Over a 12 week period, water
samples were collected weekly from the air/water lines, highspeed lines and sinks (controls) of 8
randomly chosen dental units used for nonsurgical dental procedures. Following serial dilutions,
samples were plated on R 2A agar using the spread plate technique, and incubated aerobically at
35°C for 7 days. Individual colonies that were isolated were characterized based on colony
morphology, Gram stain and oxidase test results. Representative samples of dental tubing water
lines were subsequently processed for scanning electron microscopy (SEM) and evaluated for the
presence of biofilm. In order to determine the effect of culture conditions on heterotrophic plate
counts, different culture media, plating techniques, incubation time and temperature and chlorine
neutralization were evaluated. In addition, devices such as in-line filters, independent water
reservoirs, chemical disinfection and fully autoclavable systems were evaluated for their
effectiveness at reducing the heterotrophic plate counts. Scanning electron microscopy was used
to visualize the surface of in-line filters after several minutes of use, and to determine if 0.12%
chlorhexidine injected in post filter tubing was able to prevent biofilm formation. The results of
these studies revealed that while all water samples collected from the cold water sink taps met
the standards for potable water, water collected from the warm water sink taps, the air/water lines and the highspeed lines revealed extremely high heterotrophic plate counts and failed to
meet the potable water standards in all but two instances. Although the plating techniques
analyzed did not differ significantly, the incubation conditions had very significant effects on
heterotrophic plate counts and dramatically affected the number of units that failed to meet
potable water standards. The evaluation of products which are currently being marketed as
effective ways to improve dental unit water line quality appear promising, however, aspects of
their design may still leave concerns for some practitioners.
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